Frequency assignment for multi-cell IEEE 802.11 wireless networks

ABSTRACT

A frequency planning method for use in an IEEE 802.11 wireless network is described. The frequency planning method obtains traffic load information associated with access points belonging to a multi-cell wireless network and assigns channels to the access points based on the traffic load information.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/337,694 (Attorney Docket No. 2001-0531), filedNov. 8, 2001, which is incorporated herein by reference in its entiretyfor all purposes.

BACKGROUND

[0002] The invention relates to frequency planning for wirelessnetworks.

[0003] To meet the growing demand for wireless data services, manycompanies have started deploying wireless local area networks (WLANs) inairports, hotels, convention centers, coffee shops and other locationsin which network access by the public is desirable. Many of these WLANssupport the popular IEEE standard for wireless Local Area Network (LAN)protocol, known as the IEEE 802.11 standard. The IEEE 802.11 standardincludes a medium access control (MAC) layer and several physicallayers, including a frequency-hopping spread spectrum (FHSS) physicallayer and a direct sequence spread spectrum (DSSS) physical layer.Versions of the IEEE 802.11 standard include the IEEE 802.11a standard,which describes a physical layer based on orthogonal frequency divisionmultiplexing (OFDM), and the IEEE 802.11b standard, which specifies ahigh-rate DSSS layer. Because of its maturity and low cost, IEEE 802.11bcapability has been included as standard equipment in many laptopcomputers and hand-held devices. Thus, IEEE 802.11b products make up thebulk of the installed base of IEEE 802.11 systems. The IEEE 802.11 WLANssupport data rates up to 11 Mbps, albeit over short ranges, farexceeding that to be offered by the third generation (3G) cellularwireless networks.

[0004] The IEEE 802.11 WLANs and 3G networks (or conventional cellularwireless networks) have major differences in their design at physical(PHY) and medium access control (MAC) layers to meet different needs. Ingeneral, the IEEE 802.11 design is much simpler than that of the 3Gnetwork because the IEEE 802.11 standard was devised to serve a confinedarea (e.g., a link distance of at most several hundred meters) withstationary and slow-moving users, while the 3G specifications weredeveloped for greater flexibility in terms of geographical coverage andmobility, even providing for users traveling at a high speed. As aresult, the IEEE 802.11 network can support data rates higher than thoseby the 3G networks. In addition, the cost of IEEE 802.11 equipment ismuch lower than that for 3G equipment because of the simple and opendesign of IEEE 802.11 networks, coupled with competition among WLANvendors.

[0005] In terms of operations, the 3G spectrum (such as the PersonalCommunications System (PCS) band at 1.9 GHz) is licensed and veryexpensive. As a result, every effort has been directed toward optimizingthe spectral efficiency while maintaining the quality of service interms of coverage and data rate for a limited spectrum allocation. Incontrast, the IEEE 802.11b networks operate in the unlicensedIndustrial, Scientific and Medical (ISM) band at 2.4 GHz. Since thefrequency band is free, there is apparently no pressing need to optimizethe spectral efficiency. Rather, simplicity and achieving low cost forthe equipment are more important. Despite the relatively abundantspectrum (i.e., a total of 75 MHz in the 2.4GHz Band) at the ISM band,as IEEE 802.11b networks are deployed widely, they start to interferewith each other. Such interference leads to a degradation in networkthroughput.

[0006] Frequency planning, i.e., allocation of a limited number offrequencies, for an IEEE 802.11b network is different from that for atraditional cellular network. Frequency planning techniques for cellularwireless networks are well known. In typical cellular wireless networks,such as those based on the Global System for Mobile Communications (GSM)and Enhanced Data GSM Evolution (EDGE) standards, two separate radiochannels, namely the traffic and control channels, are used to carryuser data and control traffic, respectively. For example, terminalsaccess the control channels to send control information via somecontention mechanism. After the information is successfully received andprocessed by a base station (BS), the terminal is assigned with aspecific traffic channel for transmitting its data traffic. Existingfrequency assignment or radio-resource allocation schemes were devisedmainly for such traffic channels. Such schemes seek to avoid mutualinterference among various terminals or BSs using the same frequency. Inpractical networks, there is no real-time coordination among BSs in theassignment of traffic channels to terminals in different cells. Thus,frequency assignment or radio-resource allocation is based onstatistical averages or worst cases, e.g., 90% chance of acceptable linkquality, across multiple co-channel cells. Typically, frequency planningmechanisms for traditional cellular networks tend to assign the samefrequency to cells that are a sufficient distance apart.

[0007] There is no such distinction between control and traffic channelsin the IEEE 802.11b network. Instead, all user data and controlinformation (in both directions between terminals and APs) are carriedon the same physical channel. The access to the channel by multipletransmitters is coordinated by the MAC protocol, e.g., the well-known,Carrier Sense Multiple Access (CSMA) protocol with collision avoidancefeature. Under that protocol, a transmitter can transmit only if itsenses that the channel is currently idle. As a result, even if twoclosely located APs are allocated with the same frequency channel, muchof the mutual (co-channel) interference can still be avoided by the CSMAprotocol, and the available bandwidth is shared implicitly between thetwo cells served by the two APs. In a sense, the MAC protocol providesan effective, distributed mechanism to “coordinate” the channel accessamong terminals and APs. In the worst case, both APs behave as if theyshare the same frequency. Nevertheless, the IEEE 802.11 protocol stillworks properly, thus demonstrating the robustness of its design, at theexpense of increased delay (due to backoff when sensing channel busy)and degraded network throughput.

[0008] Consequently, existing frequency allocation mechanisms that donot consider the combined effect of physical channel and MAC protocolare not directly applicable to the IEEE 802.11 networks. The MAC CSMAprotocol helps to avoid much of co-channel interference in largemulti-cell IEEE 802.11 networks, but does so at the potential expense ofnetwork performance.

SUMMARY

[0009] The invention provides for frequency planning in wirelessnetworks. Traffic load information is obtained for access pointsbelonging to a multi-cell wireless network. Channels are assigned to theaccess points based on the traffic load information.

[0010] Embodiments of the invention may include one or more of thefollowing features.

[0011] The channels may be assigned by determining, for each accesspoint, at least one set of interferers from among the other accesspoints relative to the access point. The at least one set of interferersmay be determined by determining, for each of the other access points,if any co-channel interference by the other access point is greater thanor equal to a detection threshold and, if it is determined that theco-channel interference is greater than or equal to the detectionthreshold, identifying the other access point as belonging to the set ofinterferers for the access point. The detection threshold is indicativeof a busy channel according to the CSMA protocol.

[0012] The co-channel interference may be derived from values of signalpath loss between the access point and the other access point andtransmission power of the other access point.

[0013] Particular implementations of the invention may provide one ormore of the following advantages. The frequency planning mechanismserves as a valuable tool for frequency planning of large-scalemulti-cell IEEE 802.11 WLANs by focusing on interactions among devicessuch as access points based on their traffic loads and radiopropagation. Thus, collision of signals in a frequency band that wouldotherwise occur among the APs are minimized or avoided while throughputof information is optimized. The frequency planning tool can be deployedin a number of different applications, e.g., as part of managed wirelessLAN services for business customers or, alternatively, as part of anaccess point product for an automatic and adaptive frequency planning.

[0014] Other features and advantages of the invention will be apparentfrom the following detailed description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is block diagram of a wireless network having multipleaccess points (APs).

[0016]FIG. 2 is a block diagram showing an internal architecture of anAP configured with a tool for performing a frequency assignment process.

[0017]FIG. 3 is an illustration of different classes of co-channelinterferer APs relative to a given AP.

[0018]FIG. 4 is a flow diagram of one exemplary embodiment of thefrequency assignment process (of FIG. 2).

[0019]FIG. 5 is an illustration of an exemplary frequency assignmentproduced by the frequency assignment process (of FIG. 4) for a wirelessnetwork with 7 cells and 21 APs.

[0020]FIG. 6 is an illustration of an exemplary frequency assignmentproduced by the frequency assignment process (of FIG. 4) for a wirelessnetwork with 37 cells and 111 APs.

DETAILED DESCRIPTION

[0021] Referring to FIG. 1, a wireless network 10 includes a wirednetwork 12 (e.g., a Local Area Network or “LAN”) having multiplewireless access points 14 coupled thereto. The network 10 furtherincludes wireless stations or terminals 16 associated with the differentAPs 14 to form infrastructure basic service structures (or cells) 18.The AP 14 and terminals 16 served by that AP 14 (collectively referredto as a “cell”) in a given infrastructure basic service set (BSS) 18communicate with each other over a common channel that is assigned tothe AP. In the embodiment described herein, the AP 14 and terminals 16communicate with each other according to the wireless protocol providedby the IEEE 802.11 standard. The IEEE 802.11 standard specifies themedium access control (MAC) and the physical (PHY) characteristics forWLANs. The IEEE 802.11 standard is defined in International StandardISO/IEC 8802-111, “Information Technology-Telecommunications andInformation Exchange Area Networks,” 1999 Edition, which is herebyincorporated by reference in its entirety. The APs 14 thus provide forcommunications between the terminals 16 and any devices that may beconnected to the wired network 12.

[0022] Adjacent access points (APs) in IEEE 802.11 networks can beassigned with the same channel or frequency, which is shared by thoseAPs and their associated terminals according to the multiple accessprotocol (MAC), namely, the Carrier Sensing Multiple Access withCollision Avoidance (CSMA/CA) protocol. Although the CSMA/CA protocolcan coordinate the bandwidth sharing of the same radio frequency in IEEE802.11 networks, traffic load for the APs has to be considered so thatthere is enough link capacity for the expected traffic load.

[0023] In accordance with the present invention, therefore, the network10 employs a frequency planning mechanism that considers the combinedeffects of radio propagation, the IEEE 802.11 MAC protocol and trafficload, so as to mitigate the impact of co-channel interference on theperformance of an IEEE 802.11 network.

[0024] Referring to FIG. 2, an exemplary AP 14 is shown. The AP 14includes a processor 20, coupled to the network 12 by way of a networkinterface 22. The network interface 22 permits the processor 20 to sendand receive units of data, such as packets, over the network 12 usingconventional techniques. The processor 20 is also coupled to memory 24.The memory 24 stores firmware 26 that, when executed by the processor20, causes the access point 14 to operate as described herein. Inparticular, when the AP 14 is designated to serve as a “master” AP, thefirmware 26 includes a frequency planning (or assignment) process 28that allows the AP 14 to generate channel assignments for all of the APs14 in the network 10. In an alternative embodiment, with appropriatesynchronization, each AP 14, with its own copy of the frequencyassignment software, could perform the process to determine channelassignment in a distributed manner. Also stored in memory 24 is aparameter store 30 which stores, among other information, APconfiguration 32, including channel assignment information and possiblyAP traffic load information and radio parameter data. The AP 14 can alsoinclude an I/O interface 33 to allow the AP to be connected to otherperipherals.

[0025] It will be appreciated that the functionality of the AP 14 mayreside in a computer system such as a PC or workstation, with a userinterface for manually configuring the access point with information,e.g., channel assignment, or, in the case of the AP running the channelassignment process 28, parameter data to be used by the channelassignment process, or can be connected to a management console for suchpurpose.

[0026] Alternatively, the entire channel assignment process can beinstalled and executed on a separate system such as a network managementsystem. Once the network management system or AP responsible for thechannel assignment has generated the assignment information, APconfiguration information including the channel assignment can beprovided to the APs over the network, or the APs can be configured withthe appropriate channel assignment manually.

[0027] The process 28 can be implemented as an automated process that isperformed when an initial site “layout” is being defined. At such astage, the process runs after some pre-determined time interval duringwhich initial loading information is collected. Preferably, it canexecute whenever an access point joins or is removed from the network,or whenever AP loading conditions have changed.

[0028] The AP 14 includes a wireless interface 34 that includes one ormore wireless transceivers 36. In the described embodiment, thetransceivers 36 are radio frequency (RF) transceivers. Typically, eachtransceiver 36 includes its own receiver for receiving wireless RFcommunications from a terminal, a transmitter for transmitting wirelessRF communications to a terminal, and a microprocessor to control thetransceiver. Wireless communications are received and transmitted by thetransceivers 36 via respective antennas 38, which are connected to thetransceiver. Each of the transceivers 36 and antennas 38 areconventional in configuration and operation.

[0029] Frequency planning for IEEE 802.11 networks has two distinctcharacteristics. First, according to the spectrum allocation in NorthAmerica, there are three overlapping channels for allocation in the IEEE802.11b networks and eight overlapping channels for IEEE 802.11anetworks. Thus, one has to adopt a tight frequency reuse strategy forthe 802.11 networks.

[0030] The original IEEE 802.11 specification allows for severaldifferent kinds of physical layers, including direct sequence spreadspectrum (DSSS), frequency hopping spread spectrum (FHSS) and infrared(IR). In particular, the DSSS design supports data rates of 1 and 2Mbps. Subsequently, while maintaining backward compatibility to the DSSS802.11, the IEEE 802.11b was adopted to support data rates of 5.5 and 11Mbps, operating in the 2.4 GHz ISM band. As a result, the IEEE 802.11bnetwork can support 1, 2, 5.5 and 11 Mbps, depending on radioconditions. Another extension is IEEE 802.11a, which uses a differentphysical layer known as orthogonal frequency division multiplexing(OFDM) to support data rates ranging from 6 to 54 Mbps, operating in the5.5 GHz band (the U-NII band).

[0031] Although the channel assignment technique of the process 28 isdescribed with respect to IEEE 802.11b networks, it will be understoodthat the technique can be applied to other IEEE 802.11-based networks aswell. The IEEE 802.11 MAC protocol supports the independent basicservice set (IBSS), which has no connection to wired networks (i.e., anad-hoc wireless network), as well as an infrastructure BSS, whichincludes an AP connecting to a wired network (as shown in FIG. 1). Whilethe present invention also applies to the IBSS case, only theinfrastructure BSS will be considered.

[0032] A brief description of the IEEE 802.11 MAC protocol follows. TheIEEE 802.11 specification defines five timing intervals for the MACprotocol. Two of them are considered to be basic ones that aredetermined by the physical layer: the short interframe space (SIPS) andthe slot time. The other three intervals are defined based on the twobasic intervals: the priority interframe space (PIFS) and thedistributed interframe space (DIFS), and the extended interframe space(EIFS). The SIFS is the shortest interval, followed by the slot time.The latter can be viewed as a time unit for the MAC protocol operations,although the IEEE 802.11 channel as a whole does not operate on aslotted-time basis. For IEEE 802.11b networks (i.e., with a DSSSphysical layer), the SIFS and slot time are 10 μs and 20 μs,respectively. The PIFS is equal to SIFS plus one slot time, while theDIFS is the SIFS plus two slot times. The EIFS is much longer than theother four intervals and is used if a data frame is received in error.

[0033] The IEEE 802.11 MAC supports the Point Coordination Function(PCF) and the Distributed Coordination Function (DCF). The PCF providescontention-free access, while the DCF uses the carrier sense multipleaccess with collision avoidance (CSMA/CA) mechanism for contention-basedaccess. The two modes are used alternately in time.

[0034] The DCF operates as follows. An AP (or station) with a new packetready for transmission senses whether or not the channel is busy. If thechannel is detected idle for a DIFS interval (i.e., 50 μs for IEEE802.11b networks), the AP starts packet transmission. Otherwise, the APcontinues to monitor the channel busy or idle status. After finding thechannel idle for a DIFS interval, the AP: a) starts to treat channeltime in units of slot time, b) generates a random backoff interval inunits of slot time, and c) continues to monitor whether the channel isbusy or idle. In the last step, for each slot time where the channelremains idle, the backoff interval is decremented by one. When theinterval value reaches zero, the AP starts packet transmission. Duringthis backoff period, if the channel is sensed busy in a slot time, thedecrement of the backoff interval stops (i.e., is frozen) and resumesonly after the channel is detected idle continuously for the DIFSinterval and the following one slot time. Again, packet transmission isstarted when the backoff interval reaches zero. The backoff mechanismhelps avoid collision since the channel has been detected to be busyrecently. Further, to avoid channel capture, an AP must wait for abackoff interval between two consecutive new packet transmissions, evenif the channel is sensed idle in the DIFS interval.

[0035] The IEEE 802.11 standard requires a receiver to send anacknowledge message (ACK) for each packet that is successfully received.Furthermore, to simplify the protocol header, an ACK contains nosequence number and is used to acknowledge receipt of the immediatelyprevious packet sent. That is, APs and stations exchange data based on astop-and-go protocol. The sender is expected to receive the ACK withinthe 10 μs SIFS interval after the packet transmission is completed. Ifthe ACK does not arrive at the sender within a specified ACK-timeoutperiod, or it detects transmission of a different packet on the channel,the original transmission is considered to have failed and is subject toretransmission by the backoff mechanism.

[0036] In addition to the physical channel sensing, the IEEE 802.11 MACprotocol implements a network allocation vector (NAV), whose valueindicates to each station the amount of time that remains before thechannel will become idle. All packets contain a duration field and theNAV is updated according to the field value in each decoded packet,regardless of the intended recipient of the packet. The NAV is thusreferred to as a virtual carrier sensing mechanism. The MAC uses thecombined physical and virtual sensing to avoid collision.

[0037] The protocol described above is called the two-way handshaking.In addition, the MAC also contains a four-way protocol that requires thetransmitter and receiver to exchange Request-to-Send (RTS) andClear-to-Send (CTS) messages before sending actual data, as a way toresolve the so-called hidden terminal problem.

[0038] The available number of non-overlapping channels for IEEE 802.11WLAN systems depends on the underlying PHY layer. In North America, theISM band at 2.4 GHz is divided into eleven channels for the IEEE 802.11network where adjacent channels partially overlap each other.Nevertheless, among these eleven channels, there are three completelynon-overlapping ones, separated by 25 MHz at their center frequency. Inprinciple, all eleven channels are available for allocation in a givenIEEE 802.11 network. However, it may be that overlapping channels cancause enough interference that it is not beneficial to assignoverlapping channels to APs. Therefore, only the assignment ofnon-overlapping channels is considered. The approach to frequencyplanning described herein can be extended to the allocation ofoverlapping channels with proper weighting of the overlapped spectrum,proportional to their overlaps, however.

[0039] The frequency assignment process 28 described herein focuses ontransmission by the APs because the bandwidth consumption for downlink(i.e., from AP to terminal) transmission is much higher than that foruplink (i.e., from terminal to AP) transmission for typical officeenvironment and Internet applications.

[0040] The frequency assignment process 28 takes into account theradio-path signal loss between every pair of APs in the network 10 anduses that information to define sets or classes of interferers for eachi-th AP (or “AP_(i)”). Based on the interferer classification and theexpected traffic utilization (load) associated with each AP, theeffective channel utilization as seen by each AP can be determined. Theeffective channel utilization represents the sum of the traffic load ofthe AP and that “induced” by its interferers because of channel sensing.In one embodiment, the problem of frequency planning is formulated as anon-linear zero-one integer programming problem, where one of theobjective functions is to minimize the effective utilization of the“bottleneck” channel (i.e., the AP with the most highly loaded channel).A heuristic algorithm is used to solve the problem.

[0041] For a network having M APs, indexed from 1 to M, and inaccordance with the CSMA protocol, an AP with traffic ready fortransmission determines if the assigned channel (frequency) is busy oridle. For example, if the AP detects that the received power ofco-channel interference is equal to or greater than a channel-busydetection threshold α (in units of mW), which corresponds to about −80dBm in the IEEE 802.11b standard, the channel is considered to be busy.Otherwise, it is idle.

[0042] It is possible that the channel busy status is due to a singletransmitting AP or a group of multiple APs transmitting simultaneously.For efficient frequency assignment, the interferers for each AP can beclassified as follows. Specifically, for each AP_(i), C_(i)(1) denotes aset of interfering APs where transmission by any one AP in the set cancause enough interference for AP_(i) to detect channel busy. The APs inthe set C_(i)(1) are called class-1 interferers for AP_(i). Likewise,C_(i)(2) denotes a set of pairs of two interfering APs wheretransmission by any pair of APs in the set can cause AP_(i) to sensechannel busy. The APs in C_(i)(2) are referred to herein as class-2interferers. It can be noted that transmissions by any single AP inC_(i)(2) are not sufficient to cause AP, to sense channel busy. Further,the APs in any AP pair in C_(i)(2) are not class-1 interferers to eachother.

[0043] Referring to FIG. 3, an example of interferer class definition 40for a given AP is shown. The C_(i)(1) and C_(i)(2) interferers for eachAP_(i) 16 a can be determined by measuring or estimating signal pathloss between each pair of APs in the network. Letting P_(j) and h_(ij)denote the transmission power at AP_(j) 16 b and the signal path lossfrom AP_(j) to AP_(i), respectively, the classification of AP_(j) 16 bas a C_(i)(1) interferer requires that

h _(i) P _(j) ≧a   Eq. (1)

[0044] where h_(ij)P_(j) represents, for AP_(i), the co-channelinterference contributed by AP_(j), (indicated in the figure byreference numeral 42 a) and α is the power threshold to detect channelbusy.

[0045] Similarly, where P_(m) and P_(n) denote the transmission power atAP_(m) 16 b and AP_(n) 16 c, respectively, and h_(im) and h_(in) denotethe signal path loss from AP_(m) 16 b to AP_(i) 16 a and AP_(n) 16 c toAP_(i) 16 a, respectively, the pair AP_(m) and AP_(n) belongs toC_(i)(2) if

h _(im) P _(m) +h _(in) P _(n)≧α  Eq. (2)

[0046] where h_(im)P_(m)+h_(in)P_(n) represents the co-channelinterference of the AP pair AP_(m) and AP_(n) (indicated in the figureby reference numeral 42 b).

[0047] It is assumed the transmission power in Equations (1) and (2) isfixed in this disclosure. However, the channel assignment mechanismcould be adapted to support dynamic power control as well.

[0048] It is possible to define class-3 or even higher classes ofinterferers as well. Due to the contention-oriented nature of the CSMAprotocol, however, the traffic load on each channel (i.e., theprobability of transmission at a given AP) cannot be too high. Thus, theprobability of having interferers of class-3, which require simultaneoustransmission at all three interfering APs, is much smaller relative tothat of the class-1 and class-2 interferers. Hence, for simplicity, onlyclass-1 and class-2 interferers are considered by the process 28. Theprocess 28 also takes into account AP traffic load, denoted generally byρ.

[0049] Measurement of known RF parameters such as transmission power andsignal path loss can be carried out by a dedicated hardware device, suchas a handheld measurement device, or a site survey software tool runningon a network manager console or PC, or even on the AP device itself.Many wireless LAN equipment vendors bundle such tools with their accesspoint hardware. Traffic load can also be measured or modeled bycommercially available network management software.

[0050] Once measured, modeled or estimated, such parameter data(measurements or estimates, as discussed above) is stored in the memory24 for use by the process 28.

[0051] There are a total of N (non-overlapping) channels, indexed by 1to N, available for allocation. As pointed out above, N=3 for the IEEE802.11b network for non-overlapping channels. With such a small N, it isassumed that each AP is assigned one and only one channel. An effectivechannel utilization U_(i) is defined as the fraction of time at whichthe channel can be sensed busy or is used for transmission by AP_(i).That is, $\begin{matrix}{U_{i} = {\rho_{i} + {\sum\limits_{k = 1}{{X_{ik}\left\lbrack {{\sum\limits_{j\quad {{\varepsilon Ci}{(1)}}}^{N}{\rho_{j}X_{jk}}} + {\sum\limits_{{({m,n})}\quad {{\varepsilon Ci}{(2)}}}{\rho_{m}\rho_{n}X_{mk}X_{nk}}}} \right\rbrack}.}}}} & {{Eq}.\quad (3)}\end{matrix}$

[0052] where assignment indicator (or weight) X_(ij) is equal to ‘1’ ifAP_(i) is assigned with channel_(j) and is equal to ‘0’ otherwise.

[0053] Referring to Equation (3) above, the first term ρ_(i) is theoffered traffic load for AP_(i) in terms of channel utilization withoutinterference from any source. The first summation term inside thebrackets in Equation (3) represents the total traffic load of allclass-1 interfering APs that are assigned the same channel as AP_(i). Asdiscussed earlier, according to the CSMA protocol and because of thedetection threshold α in use, AP_(i) senses channel busy when any one ofits class-1 interferers transmits on the same channel. The lastsummation term in Equation (3) represents the total traffic load of allclass-2 interferers. The interferer classes can be defined to includeoverlapping channels as well. For example, the transmission power frominterferers on overlapping channels can be weighted proportionally tothe spectrum overlap. The weight for non-overlapping channels is ‘0’,and for fully overlapping co-channel cases is ‘1’. Partially overlappingones are somewhere in between depending on their carrier frequencyoffset, filter shapes and other factors.

[0054] Channel stability is maintained (i.e., all traffic can be senteventually) by requiring that

U_(i)<S   Eq. (4)

[0055] for all AP_(i) where i=1 to M, and a threshold S is equal to avalue of 1. The value of S can be made less than 1 to account foroverhead of CSMA contention or other source of interference.

[0056] One objective function for the channel assignment is to minimizethe effective utilization of the “bottleneck” AP, that is,

minimize max {U₁, U₂ . . . , U_(m)}  Eq. (5)

[0057] over the assignment indicator {X_(ij)} subject to the constraintsof Equation (4) for all i=1 to M. Clearly, the objective function inEquation (5) is to assign channels such that the effective utilizationof the most heavily loaded AP is minimized. This results in moreresources available for the most heavily loaded AP, given offeredtraffic loads.

[0058] In one embodiment, for the channel assignment process 28 withEquation (5) as the objective function, a heuristic algorithm isutilized, as described below with reference to FIG. 4. Thus, theheuristic algorithm attempts to minimize the effective channelutilization for the bottleneck AP. The heuristic algorithm makes use ofthe following parameters: offered traffic load p_(i) and interferer setsC_(i)(1) and C_(i)(2) for each AP_(i). Preferably, the process 28 issubject to constraints of Equation (4) for all APs.

[0059] Referring to FIG. 4, the process 28 begins (step 50) bygenerating a random (initial) channel assignment for each AP, in thenetwork (step 52). This assignment is treated as the best assignmentobtained so far. The process 28 determines the effective channelutilization U_(i) for each AP_(i) based on the generated channelassignment (step 54). The process 28 identifies the AP (say, the “i-th”AP, or AP_(i)) with the highest or maximum effective channel utilization(step 56). This AP is referred to as the “bottleneck” AP. The maximumeffective channel utilization, that is, max {U_(i)}, for the assignmentis denoted by V (step 58). In case of a tie, one such AP_(i) is chosenrandomly as the “bottleneck.” For the bottleneck AP_(i), the process 28identifies its current assigned channel, say channel k (step 60). Foreach available channel n from 1 to N with n≠k and each co-channel AP(say j) in C_(i)(1) (i.e., those APs in the set that have been assignedwith channel k), the process 28 temporarily modifies the channelassignment by reassigning only AP_(j) with channel n, and recomputes themaximum effective channel utilization, denoted by W_(jn), for the newassignment (step 62). After completing such testing for all such n andj, the process 28 determines the minimum, denoted by W, from among allthe W_(jn)'s (step 64). The process 28 compares the values of W and V(step 66). If the process 28 determines that the value of W is less thanthat of V, then the process 28 replaces V by W, records the associatednew assignment as the “new” best solution (i.e., to finalize the channelchange for one AP that minimizes the objective function the most) (step70), and returns to step 54. If, at step 72, the process 28 determinesthat W and V are equal, then, with a pre-specified probability δ,preferably in the range 1>δ>O (to avoid infinite looping, as discussedlater), the process 28 replaces V by W, records the new assignment asthe best solution (step 74) and returns to step 54. If the process 28determines that W is greater than V, the process 28 saves the currentassignment and associated V value as the best solution obtained so far(that is, the current assignment is the local suboptimal assignment)(step 76). The process 28 determines if there is another randomassignment to be considered (step 78). If so, the process 28 returns tostep 52 to repeat the processing for another random assignment. If nofurther random assignments are to be considered, the process 28 selectsa final assignment as the best solution, that is, it is the channelassignment with the lowest value of V, among the local suboptimalassignments reached at step 76 (step 80). The process 28 tests the finalsolution to determine if constraints of Equation (4) for all APs aresatisfied for the final assignment (step 82). If so, the finalassignment is feasible. Otherwise, it is considered that no feasiblesolution exists for the network under consideration. After thefeasibility is tested, the process 28 terminates (step 84).

[0060] While the process 28 as illustrated in FIG. 4 may not explicitlyconsider the constraints of Equation (4), minimizing the maximum U_(i)implicitly enhances the chance of satisfying constraints of Equation (4)for all APs.

[0061] There are several characteristics of the heuristic assignmenttechnique that are worth further consideration. First, it can be shownthat the heuristic assignment technique has a loop-free property, thatis, with 1>δ>O in step 74 (FIG. 4), the heuristic algorithm does nothave infinite looping. The proof is as follows. Given that the number ofAP's M and available channels N in the system are finite, steps ofidentifying the bottleneck AP and determining W can be completed in afinite amount of time. The only possibility that the algorithm has aninfinite loop is that the steps of processing a random assignment areexecuted repeatedly without stop. Assume, preliminarily, that suchlooping can occur, that the V value after the m-th execution (iteration)is denoted by V_(m), and that δ=0 in step 74. To form the infinitelooping requires that V₁>V₂> . . . >V_(m) with m increasing towardsinfinity. With both M and N being finite, there are only a finite numberof all possible channel assignments. Since each new assignment finalizedby step 70 has a unique maximum effective channel utilization, it isthus impossible that m goes to infinity. That is, step 76 must bereached after a finite amount of processing.

[0062] Now assume that infinite looping is possible with 1>δ>0. Based onthe above argument, it is necessary to have V₁> . . . >V_(i)=V_(i+1)> .. . >V_(j)=V_(j+1)> . . . V_(m) with m going to infinity for some i andj. Since the argument above has already ruled out the possibility ofhaving subsequences of V_(i)'s of infinite length between two ‘=’ signson this list, it must contain an infinite number of ‘=’ signs. Sinceeach ‘=’ sign corresponds to an execution of the case of W=V withprobability δ, the probability of executing this step for an infinitenumber of times is thus zero. Hence, the infinite looping cannot exist.

[0063] Although it is possible to treat the case of W=V as reaching alocal optimum (like the case of W>V), numerical experience suggests thatthe case of W=V helps explore various assignments for enhanced results,especially when there are multiple bottleneck APs for the channelassignment under consideration.

[0064] Since heuristics is involved in the process 28 for the exemplaryalgorithm illustrated in FIG. 4, achieving the optimal solution is notguaranteed. It is possible, however, to quantify the quality of thesuboptimal solution generated by the algorithm. It is observed that theprocessing—in particular, steps 60, 62 and 64 (FIG. 4)—basically testsout various channel assignments to identify a better solution. As thealgorithm is executed for a given initial, random assignment, it ispossible to let Y₀, Y₁, Y₂, . . . , Y_(m), denote the (random) sequenceof the maximum effective channel utilization associated with the channelassignments under testing by step 62, with Y₀ denoting the quantity forthe initial, random assignment. Based on the Y_(i) sequence, anothersequence Z₀, Z₁, Z₂, . . . , Z_(n) is constructed as follows: (i)initialize with Z₀=Y₀ and set i=0; (ii) for each j=1, 2, . . . , m,compare Y_(j) with Z_(i); and (iii) if Z_(i)>Y_(j), then set i=i+1 andZ_(i)=Y_(j); otherwise, repeat (ii) for the next j value.

[0065] In essence, the sequence Z_(i) is constructed by examining Y_(j)one by one, starting with Z₀=Y₀ and adding Y_(j) as the last element inthe Z_(i) sequence only if Y_(j) is less than Y_(i) for all i<j (orequivalently, Y_(j) is less than Z_(i), the last element in the currentsequence). Clearly, the sequence Z_(i) is monotonic strictly decreasing.Physically, Z_(i) represents the sequence of the maximum effectivechannel utilization for an improved assignment finalized by step 70, orstep 74 (FIG. 4) that yields a maximum utilization lower than anyassignments examined by the algorithm so far in the search process.

[0066] The algorithm is repeated for a given number (say K) of initialrandom assignments. For each initial assignment, one such sequence Z_(i)(as discussed above) can be obtained. It can be noted that the sequencesassociated with different initial assignments have different lengths andare mutually independent of each other (although elements in the samesequence are dependent). Furthermore, when the algorithm eventuallystops, it is assumed that it has encountered a total of n improvedassignments (i.e., improved over those examined earlier and derived fromthe same initial assignment), which is the sum of lengths of thesequences of Z_(i) minus K.

[0067] One can view that the maximum effective channel utilization forall possible assignments for the given network has a probabilitydistribution. Allowing T_(π), to be the maximum utilization for thetop-π-fraction of assignments (e.g., the top 0.001 percentileassignments), a random assignment with its maximum utilization Z₀, gives

P[Z ₀ <T _(π)]=π  Eq. (6)

[0068] It can be proven that, if the algorithm has encountered a totalof n improved assignments at the completion of its execution, then

Q _(π)>1−(1−π)^(n−1)   Eq. (7)

[0069] where Q_(π) denotes the probability that the final suboptimalsolution generated by the algorithm falls within the top-π-fraction ofassignments. The proof is as follows. First, the case of encountering nimproved assignments for one initial, random assignment is examined. Bydefinition, $\begin{matrix}{Q_{\pi} = {{P\left\lbrack {{\min\limits_{i}Z_{i}} \leq T_{\pi}} \right\rbrack} = {1 - {{P\left\lbrack {{\min\limits_{i}Z_{i}} > T_{\pi}} \right\rbrack}.}}}} & {{Eq}.\quad (8)}\end{matrix}$

[0070] The event of (min Z_(i)>T_(π)) in the above is identical tohaving Z₀>T_(π), Z₁>T_(π), . . . , and Z_(n)>T_(π). Given that Z_(i) isa strictly decreasing (random) sequence, then

P[Z ₀ >T _(π) ΛZ ₁ >T _(π) Λ . . . ΛZ _(n) >T _(π) ]<P[Z ₀ >T _(π) ΛZ ₀¹ >T _(π) Λ . . . ΛZ ₀ ^(n) >T _(π)]  Eq. (9)

[0071] where Z₀ ^(i) is a random variable independently drawn from thesame distribution for Z₀ for i=1 to n. One can obtain Equation (9) byreplacing Z_(i) on the left hand side by Z₀ ¹ on the right side for onei at a time. Since the Z₀ ^(i) variables are independent,

P[Z ₀ >T _(π) ΛZ ₀ ¹ >T _(π) . . . Z ₀ ^(n) >T _(π) ]={P[Z ₀ >T_(π)]}^(n+1)   Eq. (10)

[0072] Using the definition in Equation (6), substituting Equation (10)into Equation (9) and then Equation (9) into Equation (8) yieldsEquation (7). The case with multiple initial random assignments isproved by exploiting the property that the sequences Z_(i) associatedwith different initial assignments are mutually independent.

[0073] The performance of the process 28 is validated by applying theprocess 28 to two settings of multi-cell networks using the IEEE 802.11air interface for which the optimal assignment is known. The settingscorrespond to settings for a seven (7) cell network and thirty-seven(37) cell network.

[0074] Referring to FIG. 5, an assignment 90 generated by the process 28for a setting that corresponds to a network with 7 cells is shown. Threeadjacent hexagon-shaped sectors 92 a, 92 b and 92 c form a cell 94. Eachsector 92 is served by an AP at the center of the cell. Each AP antennahas a beamwidth of 60′ and points toward an appropriate direction toserve the associated sector. Thus, there are 21 APs in the 7 cellnetwork, with 3 APs for each given cell co-located at the cell center,indicated by reference numeral 96.

[0075] Similarly, and referring to FIG. 6, an assignment 100 for asetting that corresponds to a network with 37 cells is shown. Threeadjacent hexagon-shaped sectors 102 a, 102 b and 102 c form a cell 104.For this setting, there are 111 APs, with 3 APs for each given cellco-located at the cell center, indicated by reference numeral 106.

[0076] The antenna gain has a parabolic shape; that is, a 3 dB droprelative to the front direction occurs at the half beamwidth angle. Anydirection beyond a threshold angle in clockwise or anti-clockwisedirection suffers a given, fixed attenuation relative to the gain at thefront direction, which is called the front-to-back (FTB) ratio. The FTBis set to be 25 dB.

[0077] It may be recalled that only the AP-to-AP interference isconsidered in the current formulation. The radio link between any pairof APs in the network is characterized by a path-loss model with anexponential of 3.5. Cell radius is assumed to be 1 Km and the path lossat 100 m from the cell center is −73 dB. Transmission power for each APantenna is 30 dBm (or 1 W). All APs have an identical amount of offeredtraffic. It will be noted that the solution generated by the process 28in this instance does not depend on the actual traffic load, but thefeasibility of the final solution does. In order to ensure that theoptimal assignment is known, shadowing and fast fading are notconsidered. In addition, the channel-busy detection threshold α is setto be 2.5e-3 μW (which corresponds to −86 dBm). As pointed out earlier,there are 3 non-overlapping channels available in the ISM band forassignment. Based on the parameter settings for both 7 and 37 cellnetworks, the optimal assignment is the traditional frequency reuse of3. That is, no adjacent sectors (APs) use the same channel.

[0078] When the process 28 is applied to the network with 7 cells and 21APs, as shown in FIG. 5, it generates the optimal channel assignmentbased on 50 random assignments. The optimal assignment 90 with channels1 to 3 assigned to the various sectors 92 a, 92 b and 92 c for each cell94 is as shown in FIG. 5.

[0079] As for the network with 37 cells and 111 APs, the process 28 wasunable to yield the obvious optimal assignment of reuse of 3, that is,without considering the boundary effect of the cell layout (which makesthe interference conditions non-uniform). The suboptimal solution forchannels 1-3 obtained from the process using 1,000 random assignments isthe assignment 100 shown in FIG. 6. It can be seen from the assignment100 that most of the sectors (APs) use a channel different from those inadjacent sectors. In the worst case, at most two adjacent sectors sharethe same channel. The process encountered and finalized a total of505,363 improved assignments. Based on the analysis set forth above,with a probability higher than 99.4%, the suboptimal solution,assignment 100, falls within the top 0.001th percentile. This result isquite acceptable.

[0080] The above two examples have uniform traffic load and uniformpropagation environments with obvious solutions and are only used toverify the correctness of the algorithm. However, for any wirelessnetwork of considerable size, the traffic load and the propagationenvironment are seldom uniform and are usually without obvious channelassignment solutions. The approach of the frequency planning process 28can easily produce a good (albeit suboptimal) channel assignmentsolution in such cases, with provable closeness to the optimal solution.Also, if the traffic load is slowly fluctuating over time, the approachcan be used to generate a series of channel assignments over time tobest accommodate the changing conditions.

[0081] Other objective functions can be used in the channel assignmentoptimization. For example, another objective function (in addition toobjective function of Equation (5)) is to minimize the overallinterference, that is,

[0082] minimize $\begin{matrix}{\sum\limits_{i = 1}U_{i}^{M}} & (11)\end{matrix}$

[0083] over the assignment indicator {X_(ij)} subject to the constraintsof Equation (4) for all i=1 to M. It can be noted that the sum of allU_(i) reflects the total effective channel utilization. Minimizing thesum tends to minimize the overall interference in the network whilemaintaining stability of each channel shared and detectable by multipleneighboring APs.

[0084] For the optimization with Equation (11) as the objectivefunction, a linear integer programming approach can be used. For a givennetwork setting, the offered load p_(i) and the interferer sets C_(i)(1)and C_(i)(2) for each AP, are known. The programming problem isnon-linear due to the cross-products of X_(ij)'s in U_(i), as defined inEquation (3). Using known techniques—for example, the techniquedescribed in the paper by W. W. Chu entitled “Optimal File Allocation ina Multiple Computer System, “IEEE Trans. On Computers, C-18, No. 10, pp.885-889, October 1969—it is possible to linearize the problem byreplacing X_(ik)X_(mk)X_(nk) by a new term Y_(ikmn). Similarly, the termX_(ik)X_(jk) is replaced by a new term Z_(ikj). The resultant problembecomes a linear integer programming problem, which has been shown to beNP-complete.

[0085] Yet another objective function is to maximize network throughput.

[0086] Other embodiments are within the scope of the following claims.For example, the above-described approach may be extended to considerone or more of the following: non-uniform transmission power by the APs;upstream traffic; overlapping channels (as discussed earlier); real-timeadaptive channel assignment to meet the fluctuation of traffic load atvarious APs over time; inclusion of path gains for stations; and specialfrequency constraints for individual AP's (e.g., AP closest to aMicrowave, WLANs of other carriers).

What is claimed is:
 1. A method for frequency planning in wirelessnetworks comprising: obtaining traffic load information for accesspoints belonging to a multi-cell wireless network; and assigningchannels to the access points based on the traffic load information. 2.The method of claim 1 wherein the step of assigning comprises:determining, for each access point, at least one set of interferers fromamong the other access points relative to the access point.
 3. Themethod of claim 2 wherein the step of determining comprises:determining, for each of the other access points, if any co-channelinterference by the other access point is greater than or equal to adetection threshold, the detection threshold indicative of a busychannel according to the CSMA protocol; and if it is determined that theco-channel interference is greater than or equal to the detectionthreshold, identifying the other access point as belonging to the set ofinterferers for the access point.
 4. The method of claim 3 wherein theco-channel interference is derived from values of signal path lossbetween the access point and the other access point and transmissionpower of the other access point.
 5. The method of claim 3 wherein the atleast one set of interferers comprises a second set of interferers, andwherein the step of determining comprises: determining, for each pair ofthe other access points, if any combined co-channel interference by suchpair is greater than or equal to a detection threshold, the detectionthreshold indicative of a busy channel according to the CSMA protocol;and if it is determined that the combined co-channel interference isgreater than or equal to the detection threshold, identifying the otheraccess points in such pair as belonging to the second set of interferersfor the access point.
 6. The method of claim 2 wherein the step ofassigning further comprises: generating random channel assignments forthe access points; determining effective channel utilization values foreach access point; modifying the random channel assignment forinterferers in the at least one set of interferers such that the highestone of the effective channel utilization values is minimized; repeatingsuch modification until the highest one of the effective channelutilization values cannot be reduced by further modification; and savingthe modified random channel assignment as a final assignment.
 7. Themethod of claim 6 wherein the step of assigning further comprises:providing the final assignment to the access points.
 8. The method ofclaim 2 wherein the step of assigning further comprises: assigningrandomly a channel to each of the access points; and computing, based onthe random channel assignment, an effective channel utilization valuefor each access point, the effective channel utilization valuerepresenting the sum of an offered load associated with the access pointand total traffic load associated with each set of interferers.
 9. Themethod of claim 8 wherein the step of assigning further comprises:determining which access point has the highest effective channelutilization value; identifying which channel is assigned to the accesspoint having the highest effective channel utilization value; and foreach access point in the first set of interferers, modifying the randomchannel assignment; recomputing the effective channel utilization valuefor the modified random channel assignment; and repeating modifying andrecomputing for each available channel other than the channel assignedto the access point having the highest effective channel utilizationvalue; determining a minimum effective channel utilization from amongthe recomputed effective utilization values; comparing the minimumeffective channel utilization and the recomputed effective channelutilization values; and replacing the highest effective channelutilization with the determined minimum effective channel utilizationand saving the modified random channel assignment as a best solution ifthe determined minimum effective channel utilization is lower than thehighest effective channel utilization.
 10. The method of claim 9 whereinthe step of assigning further comprises: with a pre-specifiedprobability, replacing the highest effective channel utilization withthe determined minimum effective channel utilization and saving themodified random channel assignment as a best solution if the determinedminimum effective channel utilization is equal to the highest effectivechannel utilization.
 11. The method of claim 6 wherein the step ofassigning further comprises: computing an effective utilization valuefor each access point based on the final assignment; and determining ifthe effective utilization value for each access point is less than avalue of one.
 12. The method of claim 1 wherein the channels comprisenon-overlapping channels.
 13. The method of claim 1 wherein the channelscomprise overlapping and non-overlapping channels.
 14. The method ofclaim 1 wherein the access points operate in accordance with the IEEE802.11 standard.
 15. The method of claim 1 wherein the step of assigningcomprises: seeking to minimize effective channel utilization of a mostheavily loaded of the access points.
 16. The method of claim 1 whereinthe step of assigning comprises: seeking to minimize total effectivechannel utilization of all access points.
 17. The method of claim 1wherein the step of assigning comprises: seeking to maximize networkthroughput.
 18. An article comprising: a storage medium having storedthereon instructions that when executed by a machine result in thefollowing: obtaining traffic load information for access pointsbelonging to a multi-cell wireless network; and assigning channels tothe access points based on the traffic load information.
 19. Anapparatus comprising: a processor; and a memory storing a computerprogram product residing on a computer-readable medium comprisinginstructions to cause a computer to: obtain traffic load information foraccess points belonging to a multi-cell wireless network; and assignchannels to the access points based on the traffic load information. 20.An access point for use in a multi-cell wireless network comprising:logic configured to obtain traffic load information for access pointsbelonging to the multi-cell wireless network; and logic configured toassign channels to the access points based on the traffic loadinformation.